Dielectric Properties of Acetamidinium Substituted Methylammonium Lead Iodide Perovskite

We study the dielectric properties of Acetamidinium lead iodide and ten percent Acetamidinium substituted Methylammonium lead iodide with pristine Methylammonium lead iodide, to understand the reason for the superior device performance of ten percent Acetamidinium substituted Methylammonium lead iodide. Measurements across a range of temperatures show that the dielectric constant in Acetamidinium lead iodide is significantly lower than Methylammonium lead iodide. One of the important findings of this report is a change in the pattern of curie-like behavior in the case of ten percent Acetamidinium substituted Methylammonium lead iodide when compared with zero percent substitution. In the case of zero percent substitution, the fall in dielectric constant after one hundred fifty kelvin is very steep, however, this curie temperature lowered to one hundred thirty eight kelvin with a slightly wider fall in the case of ten percent Acetamidinium substituted Methylammonium lead iodide due to restricted dipolar rotation of bulkier acetamidinium ion in the perovskite matrix. The fall in dielectric constant becomes even wider in pristine Acetamidinium lead iodide. So smaller exciton binding energy is not the reason for the improved performance of the mixed-cation lead iodide perovskite. It is the restricted rotation and bulkiness of Acetamidinium cation along with more H-bond that prevent the ion migration by increasing the activation energy of vacancy-mediated halide migration, which reduces interface recombination losses and improves device performance.